The present invention relates to improvements in roller presses, and more particularly to a roller press capable of interparticle crushing of granular material.
The invention of pressure comminution by interparticle crushing has been disclosed and claimed in patents such as European Patent 0 084 383 and U.S. Pat. No. 4,357,287, Schoenert. Improvements and utilization of the interparticle crushing concept have been disclosed in various patents, such as Beisner et al 4,703,897, which corresponds to European 0 084 383. In this method of comminution or grinding of brittle grinding stock such as is used on non-precomminuted cement clinker, the material is pressed under high pressure in the nip of a high pressure roller press. This causes a destruction of the particles and the production of incipient cracks in the interior of the particles. This is expressed visibly in the formation of agglomerates, commonly called scabs, that can be deagglomerated or disintegrated with comparatively little energy outlay. This process which can generally be termed pressing with the resultant interparticle crushing is accomplished with uniquely high pressing force in the nip between rollers and results in the destruction of grain structure so that a finished fine material is achieved which is distinguished by an overall low specific energy requirement.
In the operation of high pressure roller press pressing or the interparticle crushing procedure, the charging stock is supplied to the nip between oppositely driven rollers and must be drawn into the nip by friction. The individual particles in the nip are drawn into the region of the narrowest nip by friction and are mutually crushed in a product bed so that the term product bed crushing is also applied to describe the interparticle crushing procedure. The particles are mutually crushed between the material by the application of extremely high pressure, resulting in the interparticle crushing.
In apparatus which has been employed for interparticle crushing using a high pressure press, a portion of the coarse material discharged from the roller press has been recirculated. Advantageously, this material is first sifted to eliminate the already fine ground material and to discharge the coarse particles back to the roller press to again be processed.
In practice utilizing this recirculation type of grinding system, the agglomerates which are taken from the roller press are delivered back to the press by the use of a steep conveyor such as a bucket elevator which conveys the particles to a sifter arranged far above the roller press. The grits or coarse particles discharged from the sifter are then conveyed to the material delivery shaft of the roller press. In this type of arrangement, a substantial structural height is required for the equipment and such height is not always available particularly in circumstances where mills are remodeled or modernized and existing systems are rebuilt.
It is an object of the present invention to provide a recirculation type grinding system wherein the equipment requires a comparatively low structural height and involves the use of less mechanical equipment than arrangements heretofore available.
A further object of the invention is to provide an improved recirculation type of interparticle crushing roller press wherein portions are recirculated after being passed through a sifter and an improved stock feed is attained with the sifted coarse material intermixing and blending with the fresh stock in a manner to improve the grinding process.
Another object of the invention is to provide an improved structural arrangement for recirculating grinding stock in an interparticle crushing high pressure roller press wherein the equipment requires substantially less head room and wherein the construction provides for improved feeding and mixing of the recirculated stock with the fresh stock.